1. Field of the Invention
The present invention relates to an optical coherence tomography apparatus and method, and more particularly to an optical coherence tomography apparatus and method for imaging a cross-section of the eye fundus and skin.
2. Description of the Related Art
Recent years have seen a practical use of an optical coherence tomography (hereinafter referred to as an OCT) apparatus using an optical coherence technique with low coherence light. The OCT apparatus is a useful apparatus in a medical field, especially in an ophthalmic field. The OCT apparatus can provide a tomographic image of an eye fundus retinal portion and is becoming essential to the diagnosis of diseases of an eye fundus portion.
Here, the principle of the OCT will be described in brief. The low coherence light is divided into reference light and measuring light. The measuring light is incident on an object to be inspected and is reflected on a tomographic imaging region. The reflected return light is made to interfere with the reference light. The obtained interference light can be used to acquire a tomographic image of the object to be inspected. The OCT is classified into a TD (Time Domain) system and an FD (Fourier Domain). The FD-OCT system is a method for acquiring a tomographic image by performing Fourier transform on an interference signal obtained from the interference light with respect to frequency. The FD-OCT system is currently a mainstream since the tomographic image can be acquired at higher speeds than by the TD system.
Recent years have witnessed an attempt to increase the resolution in order to improve the quality of the tomographic image to be acquired. The OCT resolution is divided into a vertical resolution which is a resolution of the measuring light along an optical axis; and a lateral resolution which is a resolution in a direction perpendicular to the optical axis. The vertical resolution is important to identify a layer structure for tomographic fundus measurement using the OCT, and the layer thickness is very important to determine eye disease.
The vertical resolution in the OCT is determined primarily by the performance of the light for use in measurement. If the wavelength spectrum of the light is a Gaussian distribution, the vertical resolution is expressed by the following expression (1).
                              l          c                =                                                            (                                                                            2                      ⁢                                                                                          ⁢                                              ln                        ⁡                                                  (                          2                          )                                                                                      π                                    ⁢                                                            λ                      0                      2                                                              Δ                      ⁢                                                                                          ⁢                      λ                                                                      )                            2                        +                                          (                                  Δ                  ⁢                                                                          ⁢                                      GDL                    ·                    Δ                                    ⁢                                                                          ⁢                  λ                                )                            2                                                          (                  Expression          ⁢                                          ⁢          1                )            
Here, lc denotes a vertical resolution expressed as a half-value width of a coherence function; λ0 denotes the central wavelength of light; Δλ denotes the wavelength width of light; and ΔGDL denotes the difference in the amount of dispersion between the reference optical system and the measurement reference optical system in the OCT. The above expression assumes that the wavelength spectrum is a Gaussian distribution. If light has a spectrum which is not a Gaussian distribution, the vertical resolution is degraded from the above expression. However, the central wavelength λ0 and the light wavelength width Δλ show a similar change, and thus the above expression does not lose generality.
It is understood from expression (1) that the vertical resolution can be increased by:    (1) reducing the light central wavelength;    (2) increasing light wavelength width; and    (3) uniformizing the dispersion between the reference optical system and the measurement optical system in an interferometer.
The ophthalmologic OCT system uses a near-infrared region (with a wavelength of near 850 nm). The available wavelength band has a limit on a low wavelength side because light is absorbed in the retina. Accordingly, it is difficult to increase the vertical resolution by reducing the central wavelength in the wavelength band used by the ophthalmologic OCT system. Further, the wavelength band also has a limit on a long wavelength side because of absorption loss by vitreous body in front of the eye fundus portion and reduction in sensor sensitivity.
Thus, the vertical resolution can be increased by (2) increasing light wavelength width in consideration of the above limits. In fact, with the recent progress in the practical use of broadband low coherence light, a study has been on increased vertical resolution and clinical value by (2) increasing light wavelength width (“Ultrahigh-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation”, OPTICS EXPRESS Vol. 12, No. 11, 31 May 2004, PP 2404-2422).
Here, the dispersion compensation will be described. The OCT requires the dispersion characteristics of the reference optical path to be matched with those of the measurement optical path. The matching of the dispersion characteristics is referred to as dispersion compensation. FIG. 8 is a schematic graph illustrating two intensity profiles in the depth direction on a reflecting surface by the OCT: one profile with dispersion compensation and one without dispersion compensation. The dotted line indicates a simplified profile without dispersion compensation; and the solid line indicates a simplified profile with dispersion compensation. FIG. 8 indicates that insufficient dispersion compensation reduces the coherence function intensity indicating a resolution in the depth direction and increases the half-value width, whereby the vertical resolution is degraded.
Japanese Patent Application Laid-Open No. 2007-267927 discloses an OCT system using water for dispersion compensation. The OCT system is characterized in that a container filled with a medium with a moisture content of 70% or more is placed on the reference optical path side, and the above medium can suppress the influence of dispersion caused by an object to be measured. Japanese Patent Application Laid-Open No. 2007-267927 further discloses a technique that can deform the container to provide dispersion compensation according to the state of the object to be inspected.
A document “Ultrahigh-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation”, OPTICS EXPRESS Vol. 12, No. 11, 31 May 2004, PP 2404-2422 discloses a mathematical dispersion compensation unit using an iterative method by Hilbert transform.
In order to increase the vertical resolution using broadband light in the OCT, it is important to perform dispersion compensation over the wavelength band to be used. Unfortunately, the dispersion characteristics of an object to be measured are different for each wavelength, and thus a broader wavelength band makes it difficult to compensate dispersion by a single material, which may suppress the increase in vertical resolution.
A document “Ultrahigh-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation”, OPTICS EXPRESS Vol. 12, No. 11, 31 May 2004, PP 2404-2422 discloses an OCT configuration using broadband light. A plurality of glass materials is used to perform dispersion compensation. The materials of water and glass have greatly different dispersion characteristics in a long wavelength range (a wavelength band of about 900 nm to 950 nm). Thus, it is difficult to perform dispersion compensation on water over the broadband by the apparatus configuration disclosed in the above document.
The configuration disclosed in Japanese Patent Application Laid-Open No. 2007-267927 is characterized in that the OCT system uses water for dispersion compensation according to the object to be measured. Unfortunately, this configuration has a problem in routine use because the dispersion compensation using water involves management difficulty and quality deterioration.